1. Field of the Invention
This invention relates to a disk drive having at least one rotating memory disk. More particularly, this invention relates to a clamp used to clamp one or more rotating memory disks to a spindle motor hub to form a disk stack.
2. Description of the Related Art
Conventional disk drives typically include more than one disk stacked on a hub. Positioned between each disk is a spacer. The disks and spacers are generally referred to as a disk stack. The term disk stack also applies to a disk drives having only one disk and no spacer. The disk clamp provides a compressive load on the disk stack to hold the disks in place. The compressive load acts on the inner diameter of the disk or disks in the disk stack and is in a direction which is parallel to the axis of the hub. This compressive load may be referred to as an axial load because it acts in the axial direction.
Conventional disk clamps are available in various configurations. One known type of disk clamp uses screws passed through a circular plate and into tapped openings in the hub to provide the axial load. Unfortunately, the circular plate and screws add height to the disk stack, which is often undesirable when making smaller, shorter disk drives. In addition, the individual screws produced localized stresses in thinner disks and have the effect of causing the disks to distort at the inner diameter. The disk may actually become wavy at the inner diameter. This phenomena is often referred to a "potato chipping" because the disk's shape resembles the shape of a potato chip. Usually, each screw in the disk causes a lobe on the surface of the disk creating a non-uniform or distorted disk surface. When the disk surface is distorted in this manner, it is very difficult to maintain a uniform fly height of the transducer (i.e., read/write transducer) as the head assembly moves across the disk surface. Accordingly, when potato chipping occurs on a disk, the fly height varies and the data channel is often used to compensate for the variation in the signal from the transducer.
A second known type of disk clamp includes a bell-shaped part that operates as a spring. Typically screws are passed through openings in the center of the bell-shaped part and into a tapped opening in the hub. Unfortunately, providing a hub with enough material for a tapped opening requires height. In addition, attaching the screws at the center of the hub causes the bell-shaped part to flatten as the screws are tightened. The edges of the bell-shaped part which contact the disk during tightening move across the surface of the disk in a radially outward direction. The movement of the disk clamp with respect to the disk causes distortion, which makes the disk become conical in shape, and produces a radial load on the disk. This phenomena is often referred to as disk dropping.
A third known type of disk clamp is a heat-shrink ring which is attached to the top of the hub without the use of screws. This type of disk clamp is often referred to as a shrink-fit disk clamp. A ring is heated so that it expands and the inner diameter of the ring is greater than the outer diameter of the hub. A tool is then used to transfer the heated ring to the top of the disk stack and to apply a clamping force to the heated ring. The clamping force is maintained on the ring as it cools. Although known shrink-fit disk clamps may be designed to minimize the distortion of the disk surface and minimize the amount of height needed to accommodate the disk clamp, these shrink-fit disk clamps are more likely to slip from the hub because they are not secured by screws. Slippage generally occurs when the friction force between the disk clamp and the hub is less than the axial load applied to the disk clamp (or the residual stack load applied by the clamp). Thus, when using shrink-fit disk clamps, the coefficient of friction between the hub and the clamp must be large enough to prevent slippage. Furthermore, slippage may become more prevalent as the mass of the disk stack increases and as the spindle motor is required to withstand higher shock loads.
FIGS. 1a-b illustrate two views of a conventional N-shaped shrink-fit disk clamp 1. FIG. 1a is a top view of disk clamp 1 illustrating the ring-shape of disk clamp 1. The disk clamp 1 includes an inner diameter portion or inner leg 6, an outer diameter portion or outer leg 8, and a compliant portion or cross member 7. The cross member 7 attaches the inner leg 6 and the outer leg 8 to form the N-shape of the disk clamp 1. FIG. 1b illustrates a cross sectional view of the disk-clamp 1. The inner diameter portion includes a hub gripping portion 2. The outer diameter portion or outer leg 8 includes a disk contacting portion 4 and a free end 5. The cross member or compliant section 7 angles from the portion of the outer leg 8 near the disk contacting portion 4 to inner leg 6. The hub gripping portion 2 of the disk clamp 1 contacts a hub 3 as a result of the pre-load and the contraction of disk clamp 1 during cooling. The inner leg 6 does not contact a disk. The disk contacting portion 4 of the outer leg 8 of disk clamp 1 contacts the disk. Both the thickness of cross member 7 and an angle 9 between cross member 7 and inner leg 6 (or outer leg 8) can be varied to alter the compliance (or stiffness) of the cross member 7 such that the axial load minimizes the distortion of the disks.
As mentioned above, the coefficient of friction between hub 3 and the hub gripping portion 2 of disk clamp 1 must be high enough to prevent disk clamp 1 from slipping off of hub 3. A conventional disk clamp 1 made of an aluminum alloy and providing a clamping force of 250 lbs may require a coefficient of friction greater than 0.4 to prevent slippage from hub 3, which is also made from an aluminum alloy. In order to provide a sufficiently large coefficient of friction between the disk clamp 1 and hub 3, disk clamp 1 was deliberately oxidized (e.g. 5000-10,000 .ANG.) to form a surface coating of aluminum oxide on disk clamp 1. Although the coefficient of friction between disk clamp 1 and hub 3 is increased and the likelihood of slippage is minimized when disk clamp 1 is oxidized, the oxidation of disk clamp 1 may contaminate the disk surface and adversely affect the performance of the disk drive. More specifically, oxidized particles from disk clamp 1 fall onto the disk surface. This can result in loss of data stored in the disk drive.